I don't know of any regular passive pickups that are geared for a more "fretless" sound. What makes the sound is no frets on your fingerboard dude. If you had the money and your bass was nice enough, you could install a Piezo pickup in your bridge that picks up natural acoustic sounds from the strings. A setup like this could cost you an extra hundred bucks. Also there are external preamps that help your sound alot like Aguilar. Seymour duncan makes nice passive pickups and if u want to go a cheap route, buy a mighty mite or generic pickup.